feat: Implement background shell commands (#14849)

docs/cli/commands.md

@@ -264,6 +264,9 @@ Slash commands provide meta-level control over the CLI itself.
     modify them as desired. Changes to some settings are applied immediately,
     while others require a restart.
 
+- **`/shells`** (or **`/bashes`**)
+  - **Description:** Toggle the background shells view. This allows you to view
+    and manage long-running processes that you've sent to the background.
 - **`/setup-github`**
   - **Description:** Set up GitHub Actions to triage issues and review PRs with
     Gemini.